Picture memory (pseudomatching) in rhesus monkeys.

Pseudomatching, the selection of the correct comparison stimulus without dependence on presentation of a sample stimulus, was demonstrated in monkeys being trained ostensibly on a matching-to-sample, shock-avoidance task. Pseudomatching occurred whenever the problem sets were not fully counterbalanced for key position and correct symbol and seemed to represent memorization of specific stimulus configurations. Some animals showed the capacity to memorize hundreds of different 4-choice problems as indicated by test trials on which the samples were omitted. To prevent pseudomatching contamination of matching-to-sample behavior one must arrange full counterbalancing and randomization of stimuli and reinforcements. Pseudomatching test trials are also recommended.